The invention relates to a device for detecting contact between a material which does not conduct electricity and an electrode of an electric steel-making furnace.
In an electric steel-making furnace, steel is produced by melting scrap metal in contact with electrodes supplied with a high-intensity electric current. During operation of such a furnace, the electrodes are brought into intimate contact with the scrap charge by being moved under the control of an electric position-regulating device, so that the ends of the electrodes are immersed in the charge of scrap which is to be melted. A position-regulating device of the aforementioned kind is known and, for each electrode, produces a motion-control signal which varies inversely with the electric current flowing through the electrode.
Usually, however, the charge is not uniform and the electrode frequently comes in contact with a material in the charge which does not conduct electricity (e.g. lime, stone or wood). Such a material constitutes an electric resistance through which only a very weak current travels. The result is that the electric position regulating device generates a maximum regulating current, and thus the position regulating device operates to exert a continuous pressure on the electrode. Thereupon, the mechanical resistance of the non-conductive material to any motion of the electrode becomes such that the continuous pressure exerted by the regulating device rapidly breaks the electrode, which then has to be replaced. Electrodes are relatively expensive (over $2,250 at present) and in addition whenever an electrode is broken the furnace must be stopped, which automatically results in appreciable loss of production. The effect of such stoppages on production can easily be estimated from the fact that these stoppages normally last an hour or more and it is common for such stoppages to recur several times in a day.
Faulty operation may also be caused by an electrode which is too short or which is becoming too short as a result of wear. In that case, the arc between the electrode and the charge of scrap radiates under the furnace arc with an intensity which rapidly damages the refractory lining.
Owing to the harmful consequences of such incidents, it is desirable to detect them and remedy them without delay. The basic problem is permanently to check that an order from the regulating device is always effectively carried out and that an electrode is never held stationary when it receives an order to move from the regulating device.